The present invention relates to a pattern forming method for use in fabricating a semiconductor device, a magnetic bubble device, a superconductive device and others, and a projection aligner for carrying out the method.
As is well known, a projection exposure method is widely used for forming fine patterns such as a wiring pattern in a semiconductor device and a transfer pattern in a magnetic bubble device. Specifically, a reduction projection exposure method is very useful in forming a fine pattern. In the projection exposure method, however, the depth of focus or the focal depth of an exposure optical system is greatly dependent upon the numerical aperture of a projection lens and an exposure wavelength. The focal depth of the projection lens is inversely proportional to the square of the numerical aperture thereof. When the numerical aperture is made large to improve a resolution limit, the focal depth is decreased. Thus, it is not easy to solve difficulties due to the field curvature of the projection lens and the topography of a substrate surface. A problem which arises in forming a relatively fine pattern on a substrate surface having a difference in level (that is, a topography), has hitherto been solved by smoothing the substrate surface through the well-known multi-layer resist scheme. However, it is impossible to make completely flat the topography due to a large-area pattern by the above method, and thus the image of mask pattern inevitably becomes fuzzy at the top or bottom of the topography.
The multi-layer resist method is described in, for example, the Journal of Vacuum Science and Technology, B-1((4), 1983, pages 1235 to 1240. Further, various reduction projection aligners are shown in the Semiconductor World, June, 1984, pages 110 to 114, and other publications.
Furthermore, Japanese patent application No. JP-A-58-17,446 discloses the following method. That is, when a fine pattern is formed on a substrate through the reduction projection exposure method, an exposure operation is performed in a state that the substrate, a mask, or an optical system vibrates in the direction of optical axis of the optical system, to reduce variations in feature size of the fine pattern.
With the recent increase in the integration density of semiconductor integrated circuit, the pattern on a semiconductor integrated circuit is made fine, and the unevenness or difference in level (namely, the difference in height between the top and the bottom of the topography) of the substrate surface is made large. Accordingly, it is required to devise an appropriate countermove for the large unevenness or level difference. In a case where the projection exposure method is used for pattern formation, in order to form a fine pattern accurately on a substrate surface having large unevenness or a large level difference, it is necessary to make very large the focal depth of a reduction projection aligner used. However, when the numerical aperture of a projection lens is made large to improve a resolution limit, the focal depth of the aligner becomes small. Further, owing to the field curvature of the projection lens, the image plane of a mask is not a perfect plane. Accordingly, it is very difficult to form the image of a mask pattern accurately at the top and the bottom of the topography of an exposure area of a substrate surface.
Further, as has been already described, it is impossible to make completely flat the topography of a substrate surface due to a large-area pattern, by the multi-layer resist method. Even if the topography is made completely flat, it will be very difficult to form a fine pattern accurately on the substrate surface, since the field curvature of a projection lens makes it impossible to make the image plane of a mask pattern coincident with the substrate surface.
Further, according to the method, in which an exposure operation is performed in a state that a substrate or a mask and an optical system vibrate in the direction of an optical axis, it is possible to form the image of a mask pattern at the top and bottom of the topography of a substrate surface. However, it has been found by the inventors' investigation that when an exposure operation is performed in a state that a substrate, a mask, or an optical system vibrates in the direction of an optical axis, the image of the mask formed on the substrate is extremely small in contrast, and thus it is very difficult to form a fine pattern very accurately by developing a photoresist film which has been irradiated in accordance with the above image. The reason for this is not clear, but it is considered that when an exposure operation is performed in a state that one of the substrate, or the mask and the optical system vibrates in the direction of the optical axis, an exposure period corresponding to the upper and lower portions of the amplitude of the vibration is far shorter than an exposure period corresponding to a central portion of the amplitude, a fuzzy image of the mask is formed strongly at the top of the topography of a substrate surface, for example, and the fuzzy image reduces the image contrast in a great degree.